1. Field of the Invention
This invention relates to a semiconductor laser device that is used as a light source in optical disc apparatuses, optical communication systems, etc. More particularly, it relates to a semiconductor laser device that can attain stabilized laser oscillation for a long period of time even at high output power.
2. Description of the Prior Art
Semiconductor laser devices that can oscillate coherent light have been used as light sources in optical disc apparatuses, optical communication systems, etc. As optical disc apparatuses, there have been add-on-memory type optical disc apparatuses that achieve writing operation and erasable memory type optical disc apparatuses that achieve erasing operation. Semiconductor laser devices that are used as light sources in these optical disc apparatuses are required to produce optical output power as high as 20-40 mW. For this purpose, in recent years, high output power semiconductor laser devices have been put into practice. The high output power semiconductor laser devices are disadvantageous in that the light-emitting facet thereof tends to deteriorate. It has been reported that high output power semiconductor laser devices oscillating laser light at high output power deteriorate in inverse proportion to the fourth power of the optical output in the cases where semiconductor laser devices with the same structure are examined.
FIG. 13 shows a conventional SAM (self aligned structure by molecular beam epitaxy)-type semiconductor laser device, which is produced as follows: On an n-GaAs substrate 71, an n-Al.sub.y Ga.sub.1-y As cladding layer 72 with a thickness of 1.5 .mu.m, an undoped Al.sub.x Ga.sub.1-x As active layer 73 with a thickness of 0.08 .mu.m, a p-Al.sub.y Ga.sub.1-y As cladding layer 74 with a thickness of 0.15 .mu.m, and an n-GaAs current blocking layer 75 with a thickness of 0.8 .mu.m are successively formed by molecular beam epitaxy (wherein x&lt;y). A portion of the current blocking layer 75 is removed into a stripe with a width of 5 .mu.m in the resonating direction by an etching technique. Then, on both the remaining current blocking layer 75 and the exposed cladding layer 74, a p-Al.sub.y Ga.sub.1-y As cladding layer 76 with a thickness of 1.5 .mu.m and a p-GaAs contact layer 77 with a thickness of 0.5 .mu.m are successively formed by molecular beam epitaxy. Then, an n-side electrode 78 and a p-side electrode 79 are formed on the bottom face of the substrate 71 and the top surface of the contact layer 77, respectively, resulting in a semiconductor laser device.
When current is injected into this semiconductor laser device, only a part of the current flows into and is confined within the portion of the active layer 73 corresponding to the striped area because of the current blocking layer 75. Laser light is produced in the said portion of the active layer 73. A part of the laser light that leaks from the said portion of the active layer 73 to the outside of the said portion thereof is absorbed by the current blocking layer 75, so that a difference in the effective refractive index of the active layer arises between the said portion of the active layer 73 corresponding to the striped area and the adjacent portion of the active layer 73 corresponding to the current blocking layer 75, resulting in an optical waveguide in the portion of the active layer 73 corresponding to the striped area. The laser light that is produced in the active layer 73 is confined within the said optical waveguide, and laser oscillation can be attained in a stabilized fundamental transverse mode.
This kind of semiconductor laser device attains laser oscillation in a stabilized fundamental transverse mode for a long period of time at low optical output power. However, it is disadvantageous in that when it oscillates laser light at high optical output power, it deteriorates remarkably so that the stabilized laser oscillation cannot be continued for a long period of time. The deterioration of the abovementioned semiconductor laser device arises from a phenomenon that the portions of the current blocking layer 75 in the vicinity of the facets absorb light that leaks from the active layer 75, generating heat. When the laser device oscillates laser light at high output power, the amount of light to be absorbed in the vicinity of the facets increases and the temperature thereof rises remarkably, resulting in a deterioration of the facets of the laser device, which makes it impossible to stably oscillate laser light for a long period of time.
FIG. 14 shows a conventional VSIS (V-channeled substrate inner stripe) semiconductor laser device, which is produced as follows: On a p-GaAs substrate 81, an n-GaAs current blocking layer 82 is formed. Then, a V-channel with a width W is formed on the current blocking layer 82 in such a manner that the V-channel reaches the substrate 81 through the current blocking layer 82. On the current blocking layer 82 including the V-channel, a p-GaAlAs cladding layer 83, a GaAs or GaAlAs active layer 84, an n-GaAlAs cladding layer 85, and an n-GaAs cap layer 86 are successively formed. Current for laser oscillation is confined by the n-GaAs current blocking layer 82 and flows into only the channel. Laser light that is produced in the active layer 84 is absorbed by the n-GaAs current blocking layer 82 that is positioned outside of the V-channel, so that a difference arises in the effective refractive index between the portion of the active layer 84 corresponding to the V-channel and the other portions of the active layer 84 corresponding to the outside of the V-channel, resulting in an optical waveguide in the active layer 84. Thus, laser oscillation can be attained in a stabilized fundamental transverse mode. This VSIS laser device is disadvantageous in that although it can attain laser oscillation in a stabilized fundamental transverse mode at a low output power level, it cannot continue to oscillate laser light for a long period of time at a high output power level with extreme reliability. The reasons why the above-mentioned laser device deteriorates at a high output power level are based on the deterioration of the shoulders of the V-channel in the vicinity of the facets, which is caused by heat generation of the n-GaAs current blocking layer 82 corresponding to the shoulder portions of the V-channel that absorbs light that leaks from the optical waveguide.